Research concerning nondestructive measurements of the sugar contents of fruits has been heretofore conducted by an indirect method. That is, the color of the skin of each fruit is measured, using visible radiation. Then, the degree of maturity is estimated from the color. However, this method assumes that a correlation exists between the color of the fruit skin and the sugar content of the fruit. Therefore, this method cannot be utilized for fruits where no correlation exists between them. In addition, even for fruits in which a correlation exists between the skin color and the sugar content, the correlation is affected greatly by cultivar, the conditions of the cultivation, and other factors. Consequently, the measuring accuracy is not very high.
In recent years, NIR spectroscopy which utilizes the phenomenon of absorption of NIR radiation, and has found wide application in determination of chemical components of grain and so forth, and has been applied to the measurements of sugar contents of fruits. It has been demonstrated that this method permits accurate measurements of sugar contents of fruits having thin skins such as peaches. In this case, diffuse reflected light at the surface of the skin and at the flesh layer close to the skin is used for the measurement. Therefore, it is impossible for this method to measure the sugar contents of fruits having thick skins.